Enhance the xkey tags set on documentation pages

Enhance the xkey tags set on documentation pages

* For 'current' docs, *also* set the xkey pgdocs_current. Otherwise when
  a new major release is turned into current, the loader purge wouldn't
  be able to replace it, only replace the devel version.
* Set the pgdocs_<version> key on the release notes pages under
  /release/ (this is what triggered me to look at this)
* Set the pgdocs_all key for index pages such as /docs/ and /docs/release/. This
  will be purged every time, making sure we have an up-to-date list of
  versions.
* Set the key pgdocs_pdf for any pages that have PDF file information in
  them, so we can purge the list when new PDFs are uploaded.